It is known to have a scanning inkjet printing assembly has a print surface on which a recording medium can be arranged. For printing, a carriage supporting at least one inkjet print head is moved over the recording medium on the print surface in a scanning direction, while image-wise applying droplets of a liquid to the recording medium to form a first swath of printed dots on the recording medium. After completing such swath, which may be after a single scanning movement or after multiple scanning movements, the recording medium and the carriage are moved relative to each other in a transport direction, wherein the transport direction is transverse to the scanning direction. Then, the carriage is moved over the recording medium on the print surface in the scanning direction again, while image-wise applying droplets of the liquid to the recording medium to form a second swath of printed dots on the recording medium. Usually, the second swath is adjacent the first swath to build a printed image on the recording medium by adjacent swaths comprised of printed dots.
A known disadvantage of such a scanning inkjet printing method is that any inaccuracy in the relative movement in the transport direction will lead to inaccurate stitching of the adjacent swaths. If the movement in the transport direction is too small, the adjacent swaths will partly overlap, usually resulting in an undesired dark line in the printed image due to a local excess of a number of printed dots. If the movement in the transport direction is too large, a gap between the adjacent swaths results, which may be visible as a line with the color of the recording medium, which is usually white. So, usually, a white line becomes visible.
In order to increase the stitching accuracy, it is known to provide a sub-carriage on the carriage, wherein the sub-carriage is moveable in the transport direction relative to the carriage. The at least one print head is arranged on the sub-carriage. An inaccuracy in the relative movement in the transport direction of the carriage and the recording medium is then compensable by translating the sub-carriage relative to the carriage in the transport direction.
In practice, inaccuracies may not only be an imperfect step-wise movement. For example, if the recording medium is a web and the recording medium is step-wise moved in the transport direction, the recording medium may deform slightly resulting in a curved first swath. If a gantry moveably supporting the carriage is moved, the gantry may rotate slightly and thus the scanning direction becomes slanted relative to the first swath.
Translating the sub-carriage relative to the carriage in the transport direction cannot resolve such stitching inaccuracies. For example, with a curved swath edge, a straight edge of an adjacent swath cannot be accurately adjacent over the width of the swath. The same applies in case of a swath slanted relative to the previous swath.
It is desirable to have a scanning inkjet printing method and corresponding inkjet printing assembly improving a stitching accuracy between any two adjacent swaths.